The receiver for the digital satellite broadcast scheduled to start in the year 2000 is supposed to employ an antenna element for receiving the present analogue satellite broadcast and a down-converter for converting the output of the antenna element into BS-IF frequency, thereby receiving the digital satellite broadcast. Generally, the antenna element and the down-converter are installed outside and referred to as an outdoor unit. Hereinafter, the outdoor unit is also described as an ODU.
The receiving system for receiving the digital satellite broadcast, for example, the receiving system of CS broadcast stipulates that desirable phase noise characteristics of a local oscillator inside the down-converter used in the exclusive ODU have a phase noise (θ rms) within 4 degrees and, when the phase noise (θ rms) is within 4 degree, the receiving performance of the receiver is scarcely affected.
On the other hand, in the receiving system of the digital satellite broadcast, the existing ODU for the analogue broadcast can be used and generally the performance of the existing ODU is not good. The characteristic distribution of the phase noise of the local oscillator of the existing antenna, which was sample-studied by Association of Radio Industries and Business (abbreviated as ARIB), was as shown in FIG. 4.
At present there exists no standard concerning the phase noise for those planned as a new system. However, the phase noise characteristic thereof is expected to be the same degree as that of the above-described CS broadcast receiving system and, when the phase noise is not more than 4 degrees, the receiving performance of the receiver is not affected and no problem can be expected to arise. However, the existing ODU, especially the local oscillator having a large phase noise (θ rms) damages the receiving performance of the receiver.
Shown in FIG. 5 are the critical C/N characteristics by the phase noise (θ rms) of the local oscillator inside the down-converter of the ODU for a 8PSK (Trellis coded 8PSK) modulating signal in a burst symbol reception. Here, the system for regenerating a carrier from only the BPSK modulating signal referred to as a burst symbol signal which is intermittently transmitted is termed the burst symbol reception. Shown in FIG. 6 are critical C/N characteristics by the phase noise (θ rms) (of the local oscillator) for the 8PSK modulating signal in a continuation reception. Here, the continuation reception refers to a system for regenerating a carrier from a received signal.
In FIG. 5, the characteristics of a carrier regenerative loop are shown by a critical CNR for each of three kinds of characteristics a, b and c. The characteristic a as shown in FIG. 5 is a critical C/N where a noise bandwidth is made narrow and when the phase noise exceeds 15 degrees no reception is possible. The characteristic c as shown in FIG. 5 is a critical C/N where the noise bandwidth is made large and a reception is possible even when the phase noise is about 30 degrees. However, a fixed deterioration at a time when the phase noise is about less than 10 degrees becomes large in contrast to the characteristic a as shown in FIG. 5. The characteristic b as shown in FIG. 5 is a critical C/N which is intermediate between the case of the characteristic a as shown in FIG. 5 and the case of the characteristic c as shown in FIG. 5.
As can be seen by comparing a of FIG. 5 with FIG. 6, in case of the burst reception, the receiving performance becomes deteriorated when the phase noise becomes large depending on the characteristics of the carrier regenerative loop, while in case of the continuation reception, even with the noise bandwidth of the characteristic a as shown in FIG. 5, the fixed deterioration is lessened and the receiving performance is improved.
Now, the receiving system of the digital BS broadcast receiver will be described. In the digital BS broadcast system, a 8PSK modulation, a QPSK modulation and a BPSK modulation are adapted as modulating systems and the modulated wave thereof is time-divisionally-multiplexed and transmitted as shown in FIG. 7.
FIG. 7(a) shows the configuration of one super frame, which comprises eight frames in total. In each frame, a BPSK-modulated frame synchronous pattern as shown by the first oblique lines (32 symbols), a BPSK-modulated TMCC pattern for discriminating a transmission and multiplex configuration (128 symbols), then a BPSK-modulated super frame discrimination pattern (32 symbols), a main signal of 203 symbols, a BPSK-modulated burst symbol signal as shown by cross-oblique lines (4 symbols) and subsequently a main signal and a burst symbol signal are repeated in order, thereby configuring one frame with 39936 symbols. The main signal as shown in FIG. 7(b) is a 8PSK/QPSK/BPSK-modulating signal.
Because the modulated wave by a modulating system where the required C/N (the C/N required for demodulation) varies as the number of phases varies as eight, four and two like the 8PSK/QPSK/BPSK-modulating signal is time-divisionally-multiplexed, the BPSK-modulating signal of 4 symbols is embedded at a specific period (mainly at intervals of 203 symbols) in order to compensate for the carrier regenerative characteristics in the case where the modulating system having a number of phases is difficult to obtain reception especially at a low C/N time. The BPSK-modulating signal of the 4 symbols is termed a burst symbol signal and the system for regenerating a carrier from only the BPSK-modulating signal which is referred to as the burst symbol signal is termed the burst symbol reception as described above.
As described above, in the place where there are few phase noises, the receiving performance (the critical CNR) remains almost unchanged in case of either the burst symbol reception or the continuation reception and no problem is expected to arise. However, in the place where there are many phase noises, quite different from the continuation reception, there arises a problem for the burst symbol reception in that the critical CNR fluctuates largely according to the characteristics a, b and c of the carrier regenerative loop.
This problem will be described further in detail. By scanning a carrier frequency through the AFC circuit inserted into the carrier regenerative loop, frame synchronization is established, and when carrier regeneration is made by the burst symbol reception, Reed-Solomon error of the main signal can be checked. If the received CNR is good, the Reed-Solomon error will be eliminated and the receiving system will be switched over from the burst symbol reception to the continuation reception.
Nevertheless, when the characteristic a as shown in FIG. 5 is selected as the characteristic of the carrier regenerative loop, the Reed-Solomon error will occur in the case where the phase noise is large so that the receiving system can not be switched over to the continuation reception. As a result, the main signal is no longer regenerated indefinitely. Note that what is meant by the critical CNR as shown in FIG. 5 and FIG. 6 is the critical value where the error rate after a trellis code is decoded is 2×10−4 and which, after the Reed-Solomon is decoded, becomes error-free.
On the other hand, when the characteristic c as shown in FIG. 5 is selected as the characteristic of the carrier regenerative loop, the Reed-Solomon error will be eliminated if the received CNR is good even if the phase noise is large and the receiving system can be switched over to the continuation reception. However, as can be seen by comparing the characteristic c as shown in FIG. 5 with the characteristic as shown in FIG. 6, because the value of the critical CNR of the burst reception differs from the value of the critical CNR of the continuation reception practically irrespective of the phase noise characteristics, when the receiving system is switched over, hysteresis will occur.
However, in the situation where it is not clear which type of the ODU is to be used ultimately, it is safe to adapt the later, that is, (c) as shown in FIG. 5 for the characteristic of the carrier regenerative loop so that, whichever type of reception systems is used, it can obtain a basic reception. As a result, in spite of the fact that the digital only or the existent high performance ODU is used, a problem arises in that the receiving performance is not improved.
An object of the present invention is to provide a digital satellite broadcast receiver capable of expecting an optimum reception when the exclusive ODU or the existing high performance ODU is connected.